1. Field of the Invention
The present invention relates to optical isolators, and particularly to optical isolators with birefringent crystals which have optical axes that must have precise relative alignment to yield optimal optical performance.
2. Description of Prior Art
In present-day optical communications technology, optical signals frequently pass through a plurality of optical interfaces. At each interface, reflected signals are generated from the optical signals. If the reflected signals travel back to the signal source through the primary optical route, the source becomes unstable and noisy. Optical isolators are used to block these reflected signals from reaching the source. Ideally, optical isolators transmit all of the light of an optical signal in the forward direction, and block all of the reflected light in the reverse direction.
FIG. 1, illustrates an optical isolator 110 as disclosed in U.S. Pat. No. 5,446,813. The isolator 110 includes a first optical collimator 120, an optical isolated core 130, and a second optical collimator 140. The first optical collimator 120 collimates input optical signals from an input optical fiber 121 into the isolated core 130. The first optical collimator 120 comprises a ferrule 122 retaining the input optical fiber 121 therein, and a graded index (GRIN) lens 123. The ferrule 122 and the GRIN lens 123 are both secured into a tube 124, which in turn is further secured into a stainless steel tube 125. The second optical collimator 140 has a structure which is identical to that of the first optical collimator 120. The second optical collimator 140 is used to collimate optical signals from the isolated core 130 into an output optical fiber 141. The second optical collimator 140 is secured into a stainless steel tube 145. The isolated core 130 comprises a first birefringent crystal 131, a second birefringent crystal 133, and a Faraday rotator 132 stationed between the two crystals 131, 133. The elements of the isolated core 130 are adhered to each other, and then secured into a tube 134. The isolator 110 also has a stainless steel tube 150, with the first and the second optical collimators 120, 140 and the isolated core 130 inserted therein.
In operation, the first birefringent crystal 131 separates incident optical signals into two beams having polarization planes perpendicular to each other. Then the Faraday rotator 132 rotates the two polarized beams a specific angle xcex8, such as 45 degrees. The second birefringent crystal 133 recombines the two separated beams, and the optical collimator 140 converges the recombined beams into the output optical fiber 141. Because the Faraday rotator 132 is optically nonreciprocal, any returning optical signals from the output optical fiber 141 cannot be converged into the input optical fiber 121. As a result, the isolator 110 ensures one-way signal transmission.
Insertion loss and isolation are the two most important criteria in determining performance of the isolator 110. The most decisive factor regarding isolation is whether the angle between optical axes of the two birefringent crystals 131, 133 is equal to the rotating angle xcex8 by which the Faraday rotator 132 rotates forward singles transmitted therethrough. Furthermore, if the angle between the optical axes is equal to xcex8, insertion loss of the isolator 110 is decreased.
I t is difficult to control relative positions of the two birefringent crystal 131, 133 during assembly of the isolator 110. Accordingly, it is difficult to control precise adjustment of the angle between the optical axes of the two crystals such that the angle is equal to the rotating angle of the Faraday rotator.
Furthermore, the isolated core 130 of the isolator 110 is formed by adhering the two birefringent crystals 131, 133 and the Faraday rotator 132 together as a unit. Therefore, if the isolated core 130 is found to not meet required optical performance standards, it is necessary to discard the entire isolated core 130.
There is a need for an improved optical isolator which can overcome the disadvantages of the prior art.
Accordingly, an object of the present invention is to provide an optical isolator which allows easy and precise relative alignment of optical axes of birefringent crystals during assembly of the isolator.
Another object of the present invention is to provide an optical isolator which allows easy and precise readjustment of relative alignment of optical axes of birefringent crystals of the isolator.
To solve the problems of the prior art and achieve the objects set out above, an optical isolator in accordance with a preferred embodiment of the present invention comprises a first optical collimator, a first birefringent crystal, a Faraday rotator, a second birefringent crystal, and a second optical collimator. The first and second collimators have the same structure and configuration. Each first and second collimator comprises a ferrule, an optical fiber retained in the ferrule, and a collimating lens, all of which are secured in a tube. The first and second birefringent crystals are respectively fixed to the first and second collimators. The Faraday rotator is stationed between the first and second collimators, and fixed onto an end of the first collimator. In assembly, the first and second collimators and the Faraday rotator are all secured in a stainless steel outer tube. The second collimator is rotated within the outer tube until correct relative alignment of optical axes of the birefringent crystals is attained.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.